This invention relates generally to injection molding molds. More specifically, this invention relates to the stripping of molded parts from an injection mold featuring core lock wedges on a core portion thereof.
Injection molding basically involves the injection of a molten material, such as a plastic into a space defined between a core and a cavity of an injection mold. The core is typically an outward projection from a core portion of the mold. The cavity is typically a recess extending into a cavity portion of the mold. After injection the molten material is allowed to cool and solidify subsequent to which the core portion and cavity portion of the mold are separated in a longitudinal direction. As cooling and solidification of the part is generally accompanied by some shrinkage, the part usually shrinks onto the core from which it has to be removed or xe2x80x9cstrippedxe2x80x9d.
Various methods and apparatus are used for stripping parts, the selection of which depends upon practical considerations such as the configuration of the part to be stripped and space available. A stripping assembly which is often desired for thin wall containers and other cup shaped parts is called a stripper plate. A stripper plate, as the name suggests is a plate which lies against the core portion during molding, extends about each core, and engages an outer edge or lip of the molded part. The stripper plate may in fact define a portion of a mold face of the core. This initial position is referred to herein as the xe2x80x9cmolding positionxe2x80x9d.
The stripper plate is axially movable relative to the core by any suitable actuator connected thereto, such as a pressurized fluid displaceable piston or a linkage arm. As the stripper plate is urged away from the core portion, it presses against the part (usually a lip of the part) and urges the part off of the core. Stripper plates are desirable because they are relatively inexpensive (compared for example to pin ejection systems), reliable (compared to air ejection systems) and require minimal height and minimal changeover times.
A problem encountered with thin wall containers, particularly with relatively large ones, is a tendency of the outer edge of the cavity to flex laterally outwardly in response to the pressure of the molten material being injected, thereby causing the wall of the part to be unduly thick toward its outer edges. One way to combat this flex of the cavity is with xe2x80x9ccore lock wedgesxe2x80x9d. Core lock wedges are protrusions which extend about the core and abut against an outer perimeter of the cavity to prevent radial outward movement of the outer end of the cavity. A single xe2x80x9cwedgexe2x80x9d may be used which extends continuously about the cavity in which case the mold is referred to as having a xe2x80x9cring lock corexe2x80x9d. The present invention relates to wedge lock cores in which a plurality of spaced apart wedges are provided.
FIG. 1A is a perspective view of a prior art wedge lock core on a core part which is illustrated generally by reference 20. The core part 20 has a core 22 extending from a core face 24. Disposed about the core 22 and also extending from the core face 24 are four core lock wedges 26. Although four are shown in this illustrative example, it will be understood by persons skilled in such structures that other numbers are possible.
FIG. 1B is a section on line 2xe2x80x942 of FIG. 1A additionally showing a cavity part 30 of a mold (the cavity part 30 does not appear in FIG. 1). The cavity part 30 has a cavity 32 extending therefrom with an outer end 34 distal the cavity part 30.
FIG. 1B illustrates the core 32 and the cavity 22 in a molding configuration in which the core lock wedges 26 engage the outer ends 34 of the core 30 to resist outward lateral movement in the direction of arrows 36.
Until the present invention, a problem associated with wedge lock cores has been the inability to use a stripper plate. The presence of the core lock wedges 26 interferes with placing a stripper plate on the core face 24 and accordingly, the stripper plate is replaced with a stripper ring. FIG. 1C is a sectional view corresponding to FIG. 1B of a prior art stripper ring 40. The stripper ring 40 extends between the core 22 and the core lock wedges 26. As the stripper ring 40 does not extend to edges 28 of the core part 20, its actuation cannot be from adjacent the edges 28 as with a conventional stripper plate. Accordingly, the prior art solution has been to provide an ejector box 50 behind the core part 20 and connect the stripper ring 40 to an ejector plate 52 with ejector rods 54. The ejector plate 52 and in turn the ejector rods 54 and stripper ring 40 are moved by ejector cylinders 56 also connected to the ejector plate 52.
While the use of an ejector box 50 provides a workable arrangement, it has the disadvantage that the ejector box adds significantly to the height of the mold and accordingly, reduces the number of levels that may be provided in a stack mold.
An object of the present invention is to provide a stripper plate assembly actuatable without an ejector box for use with a wedge lock core.
A stripper assembly is provided for stripping a mold part from a core, extending from a core part of an injection mold where the core part has core lock which is disposed thereabout for engaging a corresponding mold cavity and resisting outward movement of an outer edge of the mold cavity. The core wedges are spaced apart to provide an opening between adjacent ends of the adjacent wedges.
The stripper assembly includes a stripper plate extending about a base of the core to engage a part formed on the core and is longitudinally moveable relative to the core by an actuator connected to the stripper plate, between a molding configuration in which the part is formed on the core and a stripping configuration for removing the part from the core.
The stripper plate has actuator attachment range extending through the openings between the core lock wedges to provide for attachment of the actuator thereto. The actuator is disposed laterally outwardly of the adjacent ends of the core lock wedges to move the stripper plate between the molding and stripping configurations with the core part being mounted directly to a respective part of an injection molding machine.
The actuator may include a fluid pressure response of piston slidably mounted within a bore extending into the core part adjacent at least one of the actuator attachment regions, with the piston being connected to the stripper plate.
The actuator may include two or more such pistons, each of the pistons being associated with a different one of the actuator attachment regions.
The stripper plate may have one of the actuator attachment regions at each corner thereof and respective of the pistons associated therewith. The actuator attachment regions may extend laterally beyond the confines of the injection mold and the actuator may include at least one lever arm acting between the stripper plate and the core part of the injection mold to move the stripper plate away from the core part in response to the injection mold moving from its molding to a stripping configuration.
The actuator may include two such lever arms, each of which is associated with a different one of the actuator attachment regions.
The stripper plate may have one of the actuator attachment regions at each corner thereof and a respect of the lever arms associated with each of the actuator attachment regions.
The stripper plate may include an inner plate adjacent, the core part and an outer plate adjacent the inner plate. The stripper plate is moveable by the actuator in a first stage wherein the inner and outer plates move together and further moveable by the actuator in a second stage, wherein the outer plate is moved away from the inner plate.
The first and second plates may act as an outer end of the cavity associated with the core for the forming of an outer edge of the molded part between the inner and outer plates and the core. The actuator may include first stage actuator coupled to the inner plate for moving the stripper plate in a first stage and a second stage actuator coupled to the outer plate for moving the stripper plate in the second stage.
The inner plate may include inner plate actuator attachment regions extending laterally beyond the core of the injection mold. The first stage actuator may include respective lever arms acting between the inner plate and the core part on at least two diametrically opposed of the first actuator attachment regions to move the stripper plate away from the core part in response to the injection mold moving from its molding to a stripping configuration. The outer plate may include at least two diametrically opposed second actuator attachment regions. The second stage actuator may include a respective fluid pressure responsive pistons slidably mounted within corresponding bore extending into the core part, adjacent each of the second actuator attachment regions.
A method is also provided for stripping apart from a core extending from core of injection mold where the core part has core lock wedges disposed thereabout for engaging and resisting outward lateral movement of an outer and of a corresponding mold cavity. The core lock wedges are spaced apart to provide respective openings between adjacent ends of adjacent core lock wedges. The method includes the steps of:
1. Providing a stripper plate extending about a base of the core for engaging a part formed between the core, the cavity and the stripper plate;
2. Providing the stripper plate with actuator attachment regions which extend through the respective openings between the adjacent of the core lock wedges; and
3. Applying a stripping force to the actuator attachment regions laterally outwardly of the core to move the stripper plate longitudinally relative to the core with an actuator mounted ahead of a mounting face of the core, and disposed laterally outwardly of the core.
According to one embodiment of the method, the stripper plate may further include an inner plate adjacent to the base of the core and an outer plate adjacent the inner plate. The actuator attachment regions may include first actuator attachment regions for movement of the inner plate and second actuator attachment regions for movement of the outer plate. The stripping force in step 3 may be first applied to the first actuator attachment regions to cause the inner and outer plates to move together. The stripping force in step 3 may subsequently be applied to the second actuator regions to cause the outer plate to move away from the inner plate.